I know of two return loops.
One is just up the road from me at Ely, the Adelaide Loop;
Ely West Curve
Ely West Curve (also known as the 'Adelaide loop', the 'Ely avoiding line' and the 'Ely freight loop') is a
single-track loop which branches in the up direction from Ely North Junction. The loop turns through 180 degrees to re-join the
Ely to Peterborough Line in the down (northbound) direction. Ely West Curve was laid in on 1 October 1890, to allow freight trains from the Midlands and north to run directly onto the King's Lynn and Norwich lines in the down (northbound) direction without reversal thus avoiding Ely station. Later, it was used by an increasing number of passenger trains, particularly holiday expresses to the Norfolk coast. In 1966 regular passenger traffic over the curve ceased.
[1] Currently there is one passenger train per week, in one direction only, which uses the Ely West Curve. This is the East Midland Trains 15:54 Sunday service from Norwich to Manchester Piccadilly.
The other is on the Harz Railway (Selketalbahn)....